silica gel, chemical compound. It is a colloidal form of silica, and usually resembles coarse white sand. It may be prepared by partial dehydration of metasilicic acid, H₂SiO₃. Because it has many tiny pores, it has great adsorptive power. It is used as a drying agent, as a catalyst or catalyst carrier, and in purifying various substances. Silica aerogel is fully dehydrated silica gel; it is very porous and is often used in insulation, e.g., for refrigerators.

gel: see colloid.

Hügel, Friedrich, Baron von, 1852-1925, British Roman Catholic religious writer, b. Florence; son of an Austrian diplomat. After his marriage (1873), Hügel lived in England. He wrote The Mystical Element of Religion as Studied in St. Catherine of Genoa and Her Friends (1908), a classic in the study of mysticism. Other works include Eternal Life (1912) and Essays and Addresses on the Philosophy of Religion (1921 and 1926). Through letters and essays he exerted a profound influence on the modernism movement within the Roman Catholic Church. Hügel defended the methods of modern biblical scholarship in the face of growing papal disapproval. Although a Catholic, Hügel saw divine truth in all religions, and he refused to proselytize. He regarded the adoration of God by the creature to be the essence of religion, and he stressed the values common to both the natural and the supernatural life.